
Originally Posted by
http://www.quora.com/What-is-convergent-encryption
The basic form of convergent encryption is taking your original file and calculating a hash from it. Then using this hash as the key, you encrypt the rest of the file. Finally using your password, you encrypt this hash key and store it somewhere else. This means the only way to get the password of the file is by owning the original file. If bill and ted both have the same file, they will both calculate the same hash, and the same encrypted file. Therefore since the encrypted version is the same, you only need one copy. To decrypt the file, you first decrypt the hash using your password, then the decrypt the file using the hash.